In the case of multicarrier signal transmission, the data information is transmitted on a plurality of carrier signals which have different carrier signal frequencies. Known multicarrier reception systems are DMT and OFDM (Orthogonal Frequency Division Multiplexing). Particularly in the case of mobile radio transmission, data symbols expand or overlap one another. If the delay spread of the data transmission channel is in the region of the data symbol duration, a high level of intersymbol interference may arise which makes error-free decoding impossible unless appropriate countermeasures, such as equalizers, are used. In the case of application at high data transmission rates such channel equalizers are very complex, however. Multicarrier transmission allows these drawbacks to be avoided. In the case of OFDM, the data stream to be transmitted is split into a plurality of portions and is transmitted in parallel on various signal carrier. Each subchannel may be submodulated for its part. The data transmission rate of a carrier is reduced by the parallelization. This reduces the intersymbol interference for the data transmission. The OFDM receiver performs the splitting into the subchannel or carrier signal. After filtering, sapling and demodulation, the parallel data are converted back into a serial data stream.
FIG. 1 shows a signal spectrum for a multicarrier signal transmission. The data are transmitted in a transmission frequency band which contains a multiplicity of sub-bands SBi. The sub-bands SBi normally have the same frequency bandwidth Δf. In many cases, the multicarrier system has more than 1000 sub-bands SBi. During transmission using frequency-selective multipath channels, one or more attenuation maxima, i.e. amplitude minima, may fall into the transmission band. In this case, by way of example, one sub-band SBi may be situated at an attenuation maximum while another sub-band SBi is situated at an attenuation minimum. The amplitudes of the various sub-bands SB are therefore very different. Close to an attenuation maximum, the amplitude of the useful signal is relatively small. As FIG. 1 shows, the sub-band SBi has a very small amplitude on account of a very high attenuation transmission channel.
Besides the useful signal, the receiver receives a background noise N0, which is essentially constant over the entire transmission frequency band, and external spurious signals. These external spurious signals may be signals from other signal sources or television signals, for example. The external spurious signals NF are overlaid on the background noise N0 to form a cumulative spurious signal, as shown in FIG. 1.
The received signal in the receiver is made up as follows:E=N0+NF(f)+S(f)  (1)where N0 is a largely evenly distributed background noise, NF(f) is a frequency-dependent spurious signal, and S(f) is the useful signal.
FIG. 2 shows a multicarrier signal receiver based on the prior art.
The receiver contains a tuner for tuning to the received signal, a downstream antialiasing filter AAF and an analog-digital converter for converting the received analog signal into a digital received signal. At the output of the analog-digital converter, the digital received signal is firstly supplied to a subtraction circuit SUB and to an estimation unit. The estimation unit calculates the cumulative spurious signal. The estimated cumulative spurious signal is deducted from the input signal E by the subtraction unit SUB, so that ideally just an undisturbed useful signal S remains and is processed further. The estimation unit shown in FIG. 2 performs cross correlation between the output signal from the ADC and one or more spurious signals which are to be expected.
In the case of an OFDM receiver, based on the prior art, the data are lined up symbol by symbol and are separated by one another by the guard interval. Normally, an unknown sudden phase change occurs between the data symbols. Accordingly, to subtract the estimated signal with the correct phase, the estimation unit ascertains a first cross correlation value between the received signal and a stored spurious signal which is to be expected and also a second cross correlation value between the received signal and the spurious signal to be expected which has been phase-shifted through 90°. The estimation unit then calculates the phase of the spurious signal on the basis of the cross correlation values. The calculation of this phase is severely susceptible to error.
One drawback of the conventional multicarrier signal receiver as shown in FIG. 2 is that the spurious signal needs to be estimated on the basis of magnitude and phase, which makes such estimation difficult and susceptible to error. Estimating the spurious signal becomes a very imprecise affair if the variance in the estimate result ‘measurement time’ is relatively high, e.g. because the available measurement time is too short.
The greater the discrepancy between the estimated spurious signal and the spurious signal which actually occurs, the more the bit error rate BER of the received data stream which is output by the channel decoder increases.